A standard panel lifter used, for instance, to raise gypsum sheets and hold them against ceiling joists while they are nailed or screwed to the joists, normally has a base, a mast extending upward from the base and including at least one lower section fixed in the base and at least one movable section telescoping with the lower section and having a laterally directed section face, a panel-support rack mounted at an upper end of the mast, and a crank mechanism on the base and a cable connected between the mechanism and the mast for vertically extending the mast by raising the movable mast section upward relative to the base. The rack can normally pivot at least limitedly about a horizontal axis so that the panel can be pressed against a ceiling that is inclined to the horizontal. Such devices are described in French patents 1,237,967, 2,308,760, 2,538,437, 2,552,478, and 2,623,546.
A particular hazard with such machines is that if the cable or crank mechanism fails, the panel, which can weigh in the neighborhood of 100 lbs for a 12-ft sheet of firecode gypsum board, and the rack will drop precipitously and can seriously injure the workers who are perforce underneath. The problem is compounded by the fact that it is standard to tension the cable and stress the crank mechanism considerably by pressing the panel tightly against the ceiling joists so that it can be easily attached in place.
Thus above-cited French patent 2,538,437 proposes a system where the mast is provided with an antidrop system. Two such devices are provided which act respectively between the lower and middle and between the middle and upper sections of the normally three-part mast. Such a system comprises a wedge that can pivot on the respective mast section and that is urged into a position wedged between the mast sections by a spring, while the cable, when tensioned, holds it out of contact with the mast sections. Thus if the cable breaks, the spring pushes the wedge into position to inhibit relative sliding of the respective mast sections.
The disadvantage of this system is that it often cannot provide sufficient braking force when a particularly heavy panel is being lifted, mainly because it only contacts the section it is intended to brake over a relatively limited surface area. In addition the brake is relatively expensive and complex. Furthermore since it is built in, inside the mast, it is not apparent to the user and gives no assurance that it is there and working.
In addition German utility model 295 01 130 and French patent 1,377,926 describe mast blocking systems describe systems not intended for use with crank-operated system, but instead have a pivotal latching member traversed by the math and making in effect point contact with the mast. These systems are mainly aimed at resisting horizontal stresses and are not aimed at providing antidrop action. They are not readily installed on a panel lifter.